1. Field of the Invention
The present invention relates to a fuel cell system for controlling the efficiency of a fuel cell.
2. Description of the Related Art
For example, a polymer electrolyte fuel cell employs a membrane electrode assembly which includes an anode (fuel electrode) and a cathode (air electrode), and a polymer electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is sandwiched between a pair of separators. A fuel gas flow field is formed between the anode and one of the separators, and an oxygen-containing gas flow field is formed between the cathode and the other of the separators. In use, normally, a predetermined numbers of the membrane electrode assemblies and separators are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas flow field. The fuel gas flows through the fuel gas flow field along the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the suitably humidified electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating DC electrical energy. Further, in the fuel cell, an oxygen-containing gas such as the air is supplied to the oxygen-containing gas flow field, and the oxygen-containing gas flows along the cathode for reaction. At the cathode, hydrogen ions from the anode combine with the electrons and oxygen to produce water. The produced water moves through the electrolyte membrane, and the water is also kept at the anode.
In the fuel cell system, it is preferable that the fuel cell is used in the state with high power generation efficiency. In order to increase the power generation efficiency of the fuel cell, a fuel cell system in which the load is driven by a fuel cell and an energy storage is proposed (see Japanese Laid-Open Patent Publication No. 5-182675).
In the technique disclosed in Japanese Laid-Open Patent Publication No. 5-182675, the fuel cell is operated intermittently only using a rated output having high power generation efficiency. During operation using the rated output, when the load is smaller than the rated output, the energy storage is charged by the excessive electric power. When operation of the fuel cell is stopped, electric power is supplied (discharged) from the energy storage to the load.
However, in the conventional technique, no consideration is given for the change of efficiency at the time when the power generation output is increasing, and at the time when the power generation output is decreasing.
The inventors of the present application found the cause-effect relationship about the power generation efficiency shown in FIG. 4C.
In FIGS. 4A, 4B, and 4C, for example, in a vehicle equipped with a fuel cell system as a driving source, during acceleration operation, as shown in a period from time t0 to t1, the required output (also referred to as the required electric power, the essential load, or the required load) Preq (FIG. 4A) increases continuously. As a result, the power generation output (output herein means electric power) Pfc (FIG. 4B) increases continuously. When deceleration operation is started at time t2, the required output Preq decreases instantly. As a result, the power generation output Pfc of the fuel cell decreases instantly.
In this case, as shown in FIG. 4C, during acceleration operation (time t0 to time t1), initially, the efficiency of the fuel cell (power generation efficiency) η decreases continuously from the efficiency ηs in the steady state. After time t1, the efficiency increases gradually up to the efficiency ηs in the steady state. Further, when deceleration operation is started at time t2, the efficiency η increases instantly from the efficiency ηs in the steady state. After time t2, the efficiency decreases gradually up to the efficiency ηs in the steady state.
Next, the change of efficiency η in FIG. 4C will be studied. In the polymer electrolyte fuel cell, the reaction does not occur actively in the low load state (state until time t0 in FIG. 4A where the required electric power Preq is small), and the electrolyte membrane tends to be dried. Thus, the proton conductivity in the electrolyte membrane becomes small. From this state, when the load is increased, the efficiency decreases below the efficiency ηs in the steady state (as can be seen from FIG. 4C, from time t0, the efficiency η decreases). After time t0, the fuel cell is placed in a high load state with the increased load (the required electric power Preq becomes large at time t1. After elapse of a certain period of time, reaction is induced actively, and the amount of water produced in the reaction is increased. If the high load state continues, further reaction is induced, and the operation state is improved. The proton conductivity becomes high, and the efficiency η is restored (time little bit before time t1 to time t2). When the efficiency η is restored to the efficiency ηs in the steady state, and the required load Rreq decreases sharply (time t2), if the power generation output Pfc is reduced sharply in correspondence with the change in the required load Rreq, though the fuel cell is in a suitable operating state (with high efficiency), since the power generation amount is reduced, the efficiency η is lowered again without effectively utilizing the suitable power generation state (after time t2).
FIG. 5 is a graph showing the change in the power generation efficiency η relative to the power generation output. As can be seen from FIG. 5, when the output is decreasing, the fuel cell has high power generation efficiency ηu in comparison with the power generation efficiency ηs in the steady state. When the output is increasing, the fuel cell has a power generation efficiency ηd in comparison with the power generation efficiency ηs in the steady state.